Analytical Modeling and Experimental Investigation of the Particle Scale Energy Absorption Mechanism in Laser-Based Directed Energy Deposition

Despite the tremendous application potential of advanced materials processing using laser-based directed energy deposition (DED), the reproducibility of metallic parts manufactured by this technology is limited due to the complex laser-material interactions during the material deposition process. Energy absorption and radiation by the powder stream attenuate the laser beam energy, thus affecting the actual energy delivered into the molten pool. The geometric and mechanical property irregularities in DED are directly related to the uneven energy input during continuous layers fabrication. This work reports a major advance in the particle scale energy absorption in DED as well as its relationship with the injected powder characteristics and process conditions (laser power, scanning speed, and powder feed rate). A novel simplified analytical model to analyze the effective absorptivity of continuous wave fiber laser is presented based on the stepwise heating method. The analytical model solution suggests that the effective absorptivity of the powder materials shows a slight decrease, from 8 % to 6 %, when the incident laser power increases from 250 W to 350 W. The model solution is verified using data from in-situ absorption measurements with a power meter. Theoretical calculations and experimental results showed that the energy absorption of high-density laser power in DED is not consistent, but negatively correlated with power input. Furthermore, we observed that lower powder temperature enhances energy absorption, which leads to the proposal that inverse bremsstrahlung absorption is the photophysical mechanism responsible for energy absorption by powder stream in DED additive manufacturing.